Enable "part of" for the async library.

sdk/lib/async/string_transform.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
-// part of dart.async;
+part of dart.async;
 
 abstract class _StringDecoder extends _StreamTransformer<List<int>, String> {
 
   handleData(List<int> bytes, StreamSink<String> sink) {
     var data = _carry;
     data.addAll(bytes);
     _carry = [];
     var buffer = new StringBuffer();
     int pos = 0;
     while (pos < data.length) {
@@ skipping 102 matching lines @@
         // 10xxxxxx (xxxxxx is next 6 bits from the top).
         bytes.add(((charCode >> (6 * (i - 1))) & 0x3F) | 0x80);
       }
       pos += additionalBytes + 1;
     }
     return bytes;
   }
 }